The following two methods are the principal methods used currently to determine air charge amount in an internal combustion engine. The first method is one that uses intake air flow measured by a flow rate sensor (called an “air flow meter”) disposed on the intake path. The second method is one that uses pressure measured by a pressure sensor disposed on the intake path. A method using a combination of a flow rate sensor and a pressure sensor to calculate air charge amount more accurately has also been proposed (JP2001-50090A).
However, measuring instruments such as flow rate sensors and pressure sensors sometimes have appreciably different characteristics among individual measuring instruments. Also, accuracy when calculating air charge amount from measurements taken by a flow rate sensor or a pressure sensor is affected by individual differences among constituent elements of internal combustion engines. Also, even in cases where air charge amount can be calculated correctly at the outset of use of an internal combustion engine, in some instances accuracy of calculation of air charge amount may drop due to change over time. Thus, in the past, it was not always possible to calculate accurately the air charge amount in an internal combustion engine.